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| Listing 6 . Test for a collision. |
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boolean collision =
thisSprite.isCollision(redBallSprite); |
What is a collision?
There are many ways to define and implement collision detection in game and simulation programming. In this program, a collision is deemed to haveoccurred if any portion of the rectangular redBallImage overlaps any portion of the rectangular greenBallImage . (Even though these images appear to be round, they are drawn on a transparent rectangular background.)
The isCollision method of the Sprite01 class is shown in Listing 7 .
| Listing 7 . The isCollision method of the Sprite01 class. |
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public boolean isCollision(Sprite01 other){
//Create variable with meaningful names make the// algorithm easier to understand. Can be eliminated
// to make the algorithm more efficient.float thisTop = Y;
float thisBottom = thisTop + height*scale;float thisLeft = X;
float thisRight = thisLeft + width*scale;float otherTop = other.getY();float otherBottom = otherTop + other.getHeight()*other.getScale();
float otherLeft = other.getX();float otherRight = otherLeft + other.getWidth()*other.getScale();
if (thisBottom<otherTop) return(false);
if (thisTop>otherBottom) return(false);if (thisRight<otherLeft) return(false);
if (thisLeft>otherRight) return(false);return(true);
}//end isCollision |
Methodology
This method detects a collision between the rectangular sprite object on which the method is called and another rectangular sprite object.
The methodology is to test four cases where acollision could not possibly have occurred and to assume that a collision has occurred if none of those casesare true.
Given that as background, you should be able to use a pencil and paper along with the code in Listing 7 to draw some rectangles and understand how the code in Listing 7 works.
Although I can't guarantee that the method won't call a collision when no collision actually occurred, I am pretty sure that it won't miss any collisionsthat do occur.
Process a collision
The code in Listing 8 is executed when the call to the isCollision method returns true. Therefore, this code processes a collision only when one has occurred.
The code excludes collisions between the red sprite and itself, (which is an artifact of the algorithm) . It also excludes collisions between the red sprite and blue sprites (if they exist) .
| Listing 8 . Process a collision |
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if((collision == true)&&(! thisSprite.getImage().equals(redBallImage))&&(! thisSprite.getImage().equals(blueBallImage))){
//A collision has occurred, change the color of// this sprite to blue and maybe cause it to
// die and be removed from the population.thisSprite.setImage(blueBallImage);
if(dieOnCollision){thisSprite.setLife(0);
}//end if//Cause the redBallSprite to change direction on// a random basis.
redBallSprite.setXDirection((random.nextFloat()>0.5) ? 1f : -1f);
redBallSprite.setYDirection((random.nextFloat()>0.5) ? 1f : -1f);//Cause the redBallSprite to change stepsize on a
// random basis.redBallSprite.xStep =
0.1f+random.nextFloat()*2.0f;redBallSprite.yStep =
0.1f+random.nextFloat()*2.0f;//Cause the redBallSprite to grow largerredBallSprite.setScale(redBallSprite.getScale() +
(redBallSprite.getScale()) * 0.001f);//Play a sound to indicate that a collision has
// occurred.blaster.play();
}//end if}//end for loop |
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